1. Field of the Invention
The present invention relates to an optical data recording/reproducing apparatus for optically or magneto-optically writing/reading data by using a semiconductor laser and, more particularly, to a data recording/reproducing apparatus including a high-frequency (HF) oscillator circuit for reducing noise of the semiconductor laser.
2. Description of the Related Art
Conventional optical data recording/reproducing apparatuses make use of an HF oscillator circuit for reducing noise of a semiconductor laser. An example of a laser driver circuit having the HF oscillator circuit is described in Japanese Patent Publication (KOKOKU) No. 59-9086. In this conventional example, noise is reduced by superposing an HF current on a direct current to be applied to a semiconductor laser in a read (reproducing) mode or an erase mode. In such an optical data recording/reproducing apparatus, a semiconductor laser and an HF oscillator circuit must be located closely in order to flow the HF current only into the laser and to prevent the HF current from being leaked to a portion other than the laser.
Japanese Patent Disclosure (KOKAI) No. 63-44782 discloses a semiconductor laser driving apparatus in which an HF oscillator circuit and a negative feedback circuit are arranged in a single package. This apparatus includes a circuit for preventing undesired or unwanted radiation of electromagnetic waves brought about by superposition of an HF current.
Japanese Patent Disclosure (KOKAI) Nos. 62-119743 and 63-90037 disclose an apparatus in which an HF oscillator circuit is operated to reduce noise of a semiconductor laser in a read mode, and, while the semiconductor laser is oscillating with a high output, as in a write (recording) or erase mode, the operation of the HF oscillator circuit is stopped because the influence of noise is insignificant. This arrangement makes it possible to prevent the laser from exceeding the rated output when an HF oscillation signal is superposed on a drive current, solving the problem of a short life of the semiconductor laser caused by the HF superposition.
On the other hand, radio interference is of a problem in a laser driver circuit having an HF oscillator circuit. Therefore, an HF oscillator circuit is shielded by grounding by surrounding it with a conductive package. Alternatively, in extracting an electrode from a package, such as a control signal electrode, power supply electrode, or a laser driving signal electrode, an undesired radiation preventing circuit is constituted by using, e.g., a feedthrough type capacitor and a coil. In Japanese Patent Disclosure (KOKAI) No. 63-44782, interferences caused by a leak of HF radio waves to the outside are prevented by an undesired radiation preventing circuit constituted by an LC filter consisting of a coil and a feedthrough capacitor added to a driving system of a semiconductor laser. Radio interference is prevented by this undesired radiation preventing circuit or by the shield.
Recently, demands have increasingly arisen for a high density, a high speed, and a higher transfer rate of optical data recording/reproducing apparatuses. For this reason, in an apparatus of optical modulation type in which data are written by modulating the output from a semiconductor laser, it becomes necessary to raise the modulation speed and the rising speed of the semiconductor laser output.
This undesired radiation preventing circuit, however, cuts off HF components and therefore has the following inconvenience. That is, the function of the undesired radiation preventing circuit sometimes makes it impossible to obtain a high modulation speed and a high rising speed of the semiconductor laser output during the write operation.
The apparatuses disclosed in Japanese Patent Disclosure (KOKAI) Nos. 62-119743 and 63-90037 have no problem of undesired radiation in the write and erase modes, since the HF oscillator circuit is turned off during write and erase modes. Unfortunately, a conventional write driving system of a semiconductor laser drives the semiconductor laser via an undesired radiation preventing circuit. Therefore, even an undesired radiation preventing circuit which effectively functions to prevent radiation brought about by superposition of an HF current during a read mode becomes an obstacle to a modulated write drive signal supplied by a write driving system. That is, the problem of cutting off HF components during a write mode has not been solved yet.
On the other hand, in situations where an oscillation frequency f.sub.osc of an HF oscillator circuit is close to a frequency band f.sub.w in which a semiconductor laser is driven, if a line on which an HF current is superposed is connected to a drive line for driving the semiconductor laser as in the apparatus described in Japanese Patent Disclosure (KOKAI) No. 63-90037, it is difficult to accomplish both an improvement in the transmission efficiency which is realized by improving the recording/reproducing rate by increasing the disk rotating speed, and a function of attenuating HF oscillation waves leaking from the HF oscillator circuit.
Additionally, in an apparatus which also performs a write operation, verification for checking whether data is written correctly is essential. Therefore, it is desirable that a write operation including this verification operation be completed within short time periods.
A laser driver circuit is also used in a magnetooptical recording/reproducing apparatus. In these apparatuses, as in the optical recording/reproducing apparatuses discussed above, an HF superposition scheme by which noise of a semiconductor laser is reduced in the read mode is generally used in order to ensure a high reliability by improving the signal quality.
In this HF superposition scheme, a semiconductor laser is driven by superposing an HF current of a few hundred MHz on a bias current of the semiconductor laser such that this HF current is below the threshold current, thereby making the semiconductor laser oscillate in a multi-lateral mode. This consequently reduces the coherence of the semiconductor laser, improving the system rigidity against returning light, i.e., disturbance. That is, the emitted light output when the semiconductor laser is driven with a direct current as in the read mode finely varies with disk displacement, i.e., finely varies for every disk displacement of .lambda./2 (.lambda. is the laser oscillation wavelength). This fine variation results from an alternate change of the semiconductor laser oscillation between a single-longitudinal mode, in which the light output is high, and a multi-longitudinal mode, in which the light output is low, which is attributed to changes in an external resonance mode formed between the emission surface of the semiconductor laser and an optical disk. When a high frequency is superposed, therefore, the semiconductor laser oscillates constantly in the multi-longitudinal mode to thereby suppress the fine variations.
A conventional example of a laser driver circuit using this HF superposition scheme is an apparatus in which a semiconductor laser and an HF superposition circuit are arranged adjacent to each other and placed in a shield case integrally with a laser driver circuit for driving the semiconductor laser during write and read modes.
Since, however, the laser driver circuit is provided integrally with the other components in the shield case and since the loss of the driver circuit becomes as large as about 1 W, the resulting heat raises the temperatures of the semiconductor laser and the HF oscillator circuit arranged in the shield case. The result is an unstable operation or a short life of the laser. To avoid this problem, a heat-radiating fin can be provided in the shield case. However, to effectively perform heat radiation by using the heat-radiating fin, the size of the shield case must be increased.
To solve this problem, there is a conventional apparatus in which a laser driver circuit is arranged outside a shield case.
FIG. 1 shows the outline of a conventional example which has effectuated this arrangement. A semiconductor laser (laser diode) 2 and an HF superposition circuit 4 are arranged close to and integrally with each other in a shield case 6. A laser driver circuit 8 for driving the semiconductor laser 2 for writing and reading operations, and an HF oscillation control circuit 10 for controlling the HF superposition circuit 4 are arranged outside the shield case 6. A drive line 8a of the driver circuit 8, a control line 10a extending from the HF oscillation control circuit 10 to perform on/off control of the HF superposition circuit 4, and a power line 4a for driving the HF superposition circuit 4 are connected to the individual circuits in the shield case 6 through feedthrough capacitors 12.
The HF superposition circuit 4 has an arrangement as shown in FIG. 2.
A series circuit consisting of a capacitor 14, a switch 16, and an HF oscillator circuit 18 is connected in parallel with the semiconductor laser 2. The cathode of the laser diode 2 is grounded, and its anode is connected to the driver circuit 8 via a coil 20.
The HF oscillator circuit 18 consists of a self-excited oscillator circuit constituted by transistors and the like (not shown). The HF oscillator circuit 18 superposes an HF current on the semiconductor laser 2 via the capacitor 14. That is, the HF oscillator circuit 18 superposes an HF current of a few hundred MHz on a drive current of the semiconductor laser 2 only in the read mode under the control of a switch 16 which is turned on/off by the HF oscillation control circuit 10 (to be described later). The laser driver circuit 8 drives the semiconductor laser 2 in the write and read modes; that is, the laser driver circuit 8 flows a current, which is modulated to have a changed amplitude in accordance with write data, to the semiconductor laser 2 in the write mode, and flows a constant current in the erase and read modes.
The HF oscillator circuit 18 is connected to a power supply vcc via a coil 22. The switch 16 is turned on/off by the HF oscillation control circuit 10. Note that the operation of the HF oscillator circuit 18 is controlled by on/off of the switch 16 in this example, but it is also possible to control the oscillation itself, e.g., to control the power supply of the HF oscillator circuit 18.
The semiconductor laser 2, the capacitor 14, the switch 16, and the HF oscillator circuit 18 are arranged inside the shield case 6, whereas the laser driver circuit 8 and the HF oscillation control circuit 10 are arranged outside the case 6. The feedthrough capacitors 12 are connected between a line 20a connecting the laser driver circuit 8 with the coil 20 and the shield case 6, between a line 22a connecting the coil 22 with the power supply Vcc and the shield case 6, and between a line 16a connecting the HF oscillation control circuit 10 with the switch 16 and the shield case 6.
The coils 20 and 22 and the feedthrough capacitors 12 constitute a low-pass filter for preventing a leak of HF noise from the HF oscillator circuit 18 to outside the shield case 6. The cutoff frequency of this low-pass filter is set at a value about one-tenth the frequency of the HF oscillator circuit 18 in order to prevent malfunctions occurring when electromagnetic induction noises jump to circuits or devices both inside and outside the magneto-optical recording/reproducing apparatus.
The HF oscillator circuit 10 is not limited to the above-mentioned form. As an example, Japanese Patent Disclosure (KOKAI) No. 5-267761 discloses an arrangement in which an HF oscillator circuit, an on/off control circuit for the HF oscillator circuit, and a laser driver circuit are placed in a single shield case.
Since the laser driver circuit 8 is arranged outside the shield case 6 in the above example, the problem of heat generated by the laser driver circuit 8 can be avoided. However, the semiconductor laser 2 is driven via the low-pass filter discussed above in the write and read modes. Therefore, when the recording density or the transfer rate of the magneto-optical recording apparatus is to be raised, the filter interferes with the raising of the speed of laser driving in the write mode.
That is, when the write frequency is raised to increase the write density or the transfer rate, the change rate in rise and fall of a current for driving the semiconductor laser 2 also increases to as high as, e.g., a few nanoseconds in some cases. A frequency band corresponding to this high rate is a few hundred MHz, which is close to the oscillation frequency of the HF oscillator circuit 4.
For this reason, when the low-pass filter is connected between the laser driver circuit 8 and the semiconductor laser 2, the drive current for the semiconductor laser 2 is influenced by the cutoff frequency of the low-pass filter. This sometimes decreases the amplitude or the rising speed. In particular, if the capacitance of the feedthrough capacitors 12 is about several ten pF, the inductance of the coil 20 connected in series with the semiconductor laser 2 to constitute the low-pass filter becomes around 100 nH. This decreases the rising or falling speed of the drive current for the semiconductor laser 2 or makes it difficult to perform high-speed driving of the semiconductor laser 2 by decreasing the amplitude. In addition, the power supply voltage of the laser driver circuit 8 must also be increased in performing high-speed driving including this low-pass filter. This introduces an additional problem of an increase in the consumption power of the driver circuit.